1. Field of the Invention
The present invention relates to a light output control circuit for controlling the light output of a light-emitting device, such as a semiconductor laser (laser diode—LD) or a light-emitting diode (LED), used in optical communication and other applications.
2. Description of the Related Art
Generally, in an apparatus, such as an optical transmission apparatus, that uses a light-emitting device, the light output of the device must be controlled to a specified value. On the other hand, the light-producing efficiency of a light-emitting device such as an LD has a strong temperature dependence, and furthermore, the efficiency changes with age. Accordingly, to control the light output to a constant value under all possible conditions, the current supplied to the light-emitting device must be controlled to a proper value. Traditionally, negative feedback control is used to control the light output of a light-emitting device to a constant value. FIG. 1 shows a prior art light output control circuit for controlling the light output of a light-emitting device. The operation of this circuit will be described below.
A drive current modulated with data in an LD driving circuit 10 is supplied to a light-emitting device 12. The peak value of this drive current (hereinafter called the drive current value) is controlled in proportion to a digital value input to a D/A conversion circuit 14. The digital value is supplied from a counter 16 placed directly before it. That is, the drive current value obtained is proportional to the counter value.
A photodiode (PD) 18 produces a monitor current proportional to the amount of light emitted by the light-emitting device (LD), and a monitor section 20 converts the monitor current value into a voltage value and holds its peak value. The output of the monitor section 20 is compared with a reference value (voltage) 24 using a comparator 22, and the count value of the counter 16 is manipulated in accordance with the result of the comparison. That is, if the monitor output is smaller than the reference value, the counter value is incremented by 1 to increase the drive current value; on the other hand, if the monitor output is larger than the reference value, the counter value is decremented by 1 to decrease the drive current value. With this operation, the light output is controlled at a constant value with respect to the reference value, using negative feedback control.
At this time, the drive current value is controlled with an accuracy equivalent to the resolution determined by the least significant bit (LSB) of the D/A conversion circuit 14. For example, when a 10-bit D/A conversion circuit is used, a resolution of 210=1024 is obtained. When controlling the output current within a range of 10 to 100 mA using this circuit, a current mirror circuit in the LD driving circuit 10 is designed so that 1 LSB corresponds to 0.1 mA, with digital values 100 to 1000 corresponding to drive current values 10 to 100 mA. In this case, since, at the minimum value 10 mA of the drive current, the current 0.1 mA per LSB corresponds to 1%, the above circuit can control the drive current with an accuracy of about 1% (the light output is also proportional to it). The prior art has achieved highly precise control using the above-described circuit.
The problem of the prior art output control circuit is that it takes time for the light output to reach the desired value, that is, the rise time is long.
FIG. 2 shows the operating characteristic of the prior art light output control circuit. Time is plotted along the abscissa and the drive current value along the ordinate. The value of the counter circuit is reset at power on. When the above-described feedback is activated, the counter value is updated by incrementing or decrementing by 1 at a time, depending on the result of the comparison made between the output of the monitor section 20 and the reference value 24, and the drive current value is updated accordingly, causing the drive current to change in a steplike manner. At this time, the drive current value changes in steps of 0.1 mA which is proportional to the current corresponding to the least significant bit (LSB) of the D/A conversion circuit. When the drive current value reaches the target value, the drive current value oscillates between two values, one above and the other below the target value, and a light output value stabilized within this range is obtained.
In this prior art, since the digital value is incremented or decremented in steps of 1 after power on until reaching the target value, if the difference between the target value and the current value (digital value) at power on is large, it takes a large number of steps until the current rises to the desired value, and the rise time thus becomes long.
In access optical communication systems that have recently entered commercial service, a burst transmission method that divides data into cells and transmits them in bursty manner has to be employed as the method of transmission between the subscriber and the network. FIG. 3 shows the operation when the prior art light output control circuit is applied for burst transmission applications, to explain the problem of the prior art. As in FIG. 2, the time is plotted along the abscissa and the drive current value along the ordinate.
When starting up a burst transmission apparatus, only a short period of time of a few microseconds allocated to initial starting cells is allowed for the light output to rise to the target value. Accordingly, if the prior art light output control circuit were used for burst transmission, the light output could not rise fully during the starting cell period and hence could not be stabilized before the output of ordinary communication cells. Consequently, the prior art light output control circuit cannot be used for such applications.
For example, when the counter is started at an initial value 0 and is operated to count up until a maximum drive current value of 100 mA is obtained using, for example, a 10-bit D/A conversion circuit, in the worst case the counter would have to be updated 1024 times before the output is stabilized.
Thus the prior art has had the problem that it cannot be used for burst transmission applications because the rise time is long.
In the case of continuous transmission also, data cannot be transmitted during the start-up period. If the rise time of the optical module transmitter is long, the problem is that it takes time to start up the entire system. This also limits the time margin of the system or slows the system power up.
Another problem that arises when the prior art of FIG. 1 is used for burst transmission concerns the control when no cells exist. In the prior art of FIG. 1, since the control of the counter 16 is performed at all times regardless of the presence or absence of a signal, the time constant of the monitor section 20 must be made sufficiently long in order to maintain stable control even when there are no cells. Increasing the time constant, however, leads to the problem of slow response.
A further problem with the prior art of FIG. 1 concerns very small fluctuations occurring after the control has reached a steady-state condition. As shown in FIG. 2, in the prior art light output control circuit, after the start up of the light output control circuit is completed and steady state is reached, the light output value continually oscillates between two values around the reference value. Usually, the circuit is designed to control the light output within specified limits despite the variations between the two values, but the light output value cycles through unnecessary changes.
Especially, in burst transmission, the time required for the light output to stabilize must be shortened; to achieve this, the feedback loop speed must be increased, but if the feedback is always performed at high speed, unnecessary updates will be repeated. This problem will result in increased error rate at the optical signal receiving end.
Furthermore, in the prior art light output control circuit, when the light output control circuit updates its light output value, the digital code input to the D/A conversion circuit 14 changes, and at this time, a spike called a glitch occurs. This glitch appears in the drive current, causing waveform distortion. This problem also results in increased error rate at the optical signal receiving end.